As is well-known in the industry, hydrocarbons are recovered from subterranean reservoirs by drilling a borehole (wellbore) into the reservoir. Such boreholes are commonly drilled using a rotating drill bit attached to the bottom of a drilling assembly (which is commonly referred to in the art as a bottom hole assembly or a BHA). The drilling assembly is commonly connected to the lower end of a drill string including a long string of sections (joints) of drill pipe that are connected end-to-end via threaded pipe connections. The drill bit, deployed at the lower end of the BHA, is commonly rotated by rotating the drill string from the surface and/or by a mud motor deployed in the BHA. Mud motors are also commonly utilized with flexible, spoolable tubing referred to in the art as coiled tubing. During drilling a drilling fluid (referred to in the art as mud) is pumped downward through the drill string (or coiled tubing) to provide lubrication and cooling of the drill bit. The drilling fluid exits the drilling assembly through ports located in the drill bit and travels upward, carrying debris and cuttings, through the annular region between the drilling assembly and borehole wall.
In recent years, directional control of the borehole has become increasingly important in the drilling of subterranean oil and gas wells, with a significant proportion of current drilling activity involving the drilling of deviated boreholes. Such deviated boreholes often have complex profiles, including multiple doglegs and a horizontal section that may be guided through thin, fault bearing strata, and are typically utilized to more fully exploit hydrocarbon reservoirs. Deviated boreholes are often drilled using downhole steering tools, such as two-dimensional and three-dimensional rotary steerable tools. Certain rotary steerable tools include a plurality of independently operable blades (or force application members) that are disposed to extend radially outward from a tool housing into contact with the borehole wall. The direction of drilling may be controlled, for example, by controlling the magnitude and direction of the force or the magnitude and direction of the displacement applied to the borehole wall. In such rotary steerable tools, the blade housing is typically deployed about a rotatable shaft, which is coupled to the drill string and disposed to transfer weight and torque from the surface (or from a mud motor) through the steering tool to the drill bit assembly. Other rotary steerable tools are known that utilize an internal steering mechanism and therefore don't require blades (e.g., the Schlumberger PowerDrive rotary steerable tools).
Directional wells are also commonly drilled by causing a mud motor power section to rotate the drill bit through a displaced axis while the drill string remains stationary (non-rotating). The displaced axis may be achieved, for example, via a bent sub deployed above the mud motor or alternatively via a mud motor having a bent outer housing. The bent sub or bent motor housing cause the direction of drilling to deviate (turn), resulting in a well section having a predetermined curvature (dogleg severity) in the direction of the bend. A drive shaft assembly deployed below the power section transmits downward force and power (rotary torque) from the drill string and power section through a bearing assembly to the drill bit. Common drive shaft assemblies include a coaxial shaft (mandrel) deployed to rotate in a housing.
The non-rotating sections (e.g., the above described housings) commonly include MWD and/or LWD sensors, electronic components and controllers, and electrical actuators (e.g., solenoid actuated valves and switches used to control steering blades). In the above described drilling assemblies a gap typically exists between the rotating and non-rotating sections (e.g., between the shaft and housing). Thus electrical power must be stored and/or generated in the non-rotating section or transferred across the gap from the rotating section to the non-rotating section. Moreover, in order to provide electronic communication between the rotating and non-rotating sections, data must also be transferred back and forth across the gap.
Slip ring assemblies are commonly utilized to transmit electrical power and electronic data across the gap between rotating and non-rotating tool sections. While slip ring assemblies have been used commercially, they can be problematic. For example, slip ring assemblies typically include a number of small components that must be precisely aligned and can therefore be difficult to assemble in the limited physical space between a shaft and sleeve. This difficulty is particularly evident in small diameter (slim) tool embodiments.
Slip rings have also been known to fail in service. Such failures are costly in that they commonly result in a loss of communication with the tool and the need to trip out of the borehole. For example, the failure of slip ring seals can cause a tool failure. Loss of electrical contact between the slip ring contact members (e.g., due to wear) is also a known cause of tool failure. The electrical performance of slip rings is also susceptible to both long term and short term degradation when exposed to oil. Furthermore, when used with heavier grade lubricating oils, liftoff of the contacts may occur. Interruption of the electrical current can then cause burning of the oil and contamination to the contacts.
Owing to the demand for smaller diameter and less expensive rotary steerable tools (and downhole tools in general) and to the increased demand for electrical power in such tools, there is a need for improved slip ring assemblies.